The present invention relates to an ink jet recording apparatus.
An ink jet head for discharging ink by a piezoelectric effect of a piezoelectric element has been used in the art in a recording apparatus such as a printer, a facsimile, and a copier. An ink jet head of this type includes pressure chambers filled with ink, nozzles communicated to the pressure chambers, and piezoelectric actuators for applying a pressure on the ink in the pressure chambers. The recording apparatus is provided with a driving circuit for supplying a driving signal to the piezoelectric actuators. When discharging ink, a driving signal is supplied from the driving circuit to the piezoelectric actuator. The piezoelectric actuator receiving the driving signal applies a pressure on the ink in the pressure chamber so as to push out the ink through the nozzle. In this way, an ink droplet is discharged from the nozzle and lands on recording paper, thus forming a predetermined image, or the like, on the recording paper.
While a pulse signal is commonly used as the driving signal, the pulse signal needs to have a sufficient pulse width and a sufficient peak value in order to discharge ink. A pulse signal in which the pulse width or the peak value is too small is insufficient as an ink discharging signal. However, techniques for actively using such a small pulse signal for the purpose of improving the ink discharging performance have been proposed in the art. Specifically, such techniques use minute pulse signals such that ink is not discharged, as auxiliary pulse signals, in addition to ink discharge pulse signals for the purpose of improving the ink discharging performance.
For example, Japanese Laid-Open Patent Publication No. 11-277744 discloses a technique for driving an ink jet head capable of discharging three different types of ink droplets for forming small, medium and large dots, respectively, wherein an auxiliary pulse signal is applied so as to replace ink in the vicinity of a nozzle opening whose viscosity has been increased with ink in the pressure chamber having an appropriate viscosity only during a printing cycle in which ink is not to be discharged and a printing cycle in which a medium dot is to be formed.
Japanese Laid-Open Patent Publication No. 5-16359 discloses a technique for changing the discharged ink volume (i.e., the volume of ink discharged in a single shot), in which an auxiliary pulse signal is applied before the application of an ink discharge pulse signal, and then an ink discharge pulse signal is applied so that the period thereof is matched with that of the residual pressure wave created by the auxiliary pulse signal.
However, in the ink jet head disclosed in Japanese Laid-Open Patent Publication No. 11-277744, the auxiliary pulse signal for preventing an increase in viscosity is applied selectively during some of the printing cycles so that the auxiliary pulse signal does not hinder the ink discharging operation. Specifically, if there is only a short interval between an auxiliary pulse signal and an ink discharge pulse signal, the residual vibration caused by the auxiliary pulse signal affects the ink discharge. Therefore, the application of the auxiliary pulse signal is restricted to a printing cycle in which ink is not to be discharged and a printing cycle for forming a medium dot, in which a long interval is ensured between the auxiliary pulse signal and the ink discharge pulse signal. This requires a circuit for turning ON/OFF the application of the auxiliary pulse signal, thus resulting in a complicated control and increasing the cost of the control circuit.
In the ink jet head disclosed in Japanese Laid-Open Patent Publication No. 5-16359, the auxiliary pulse signal is a signal that is applied for the purpose of changing the discharged ink volume, and the auxiliary pulse signal is not applied during a printing cycle in which no ink discharge pulse signal is applied. Therefore, ink in the vicinity of the opening of a nozzle through which ink is not discharged for a number of printing cycles may have a considerably high viscosity, in which case it difficult to appropriately discharge an ink droplet from the nozzle in the next ink discharging operation. This leads to problems such as dot diameter variations and a failure to discharge ink.
The present invention has been made in view of the above, and has an object to improve the discharged ink volume and to prevent an increase in the viscosity of ink in the vicinity of a nozzle opening by using an inexpensive configuration.
An ink jet recording apparatus of the present invention includes: a head body provided with a nozzle and a pressure chamber, which is communicated to the nozzle and is filled with ink; an actuator provided in the head body and including a piezoelectric element and an electrode for applying a voltage across the piezoelectric element for applying a pressure on the ink in the pressure chamber; and a driving circuit for supplying an actuator driving signal to the electrode of the actuator, wherein: in every printing cycle, the driving circuit always supplies an auxiliary pulse signal for driving the actuator to a degree such that the ink is not discharged; and if an ink discharge instruction signal instructing an ink discharge is received, the driving circuit supplies, after the auxiliary pulse signal is supplied, an ink discharge pulse signal for driving the actuator so that the ink is discharged and so that an ink meniscus vibration in the nozzle is resonant with that caused by the auxiliary pulse signal.
Note that the term xe2x80x9cresonancexe2x80x9d is used herein in its broad sense to mean not only resonance at the resonance point, but also resonance within a predetermined range from the resonance point.
In this way, since the auxiliary pulse signal is always supplied irrespective of whether or not ink is to be discharged, it is possible to suppress an increase in the viscosity of ink even for those nozzles through which ink is not discharged for a long period of time. Moreover, since the ink meniscus vibration caused by the auxiliary pulse signal is resonant with that caused by the ink discharge pulse signal, the amount of flexural deformation of the actuator when discharging ink is increased from that in a case where the auxiliary pulse signal is not supplied. Therefore, the discharged ink volume is increased. In a case where the auxiliary pulse signal is applied after the application of the ink discharge pulse signal, it is necessary to provide a time interval after the application of the auxiliary pulse signal so that the residual vibration caused by the auxiliary pulse signal does not affect the following printing cycle. With this recording apparatus, however, the auxiliary pulse signal is applied before the application of the ink discharge pulse signal. Therefore, it is not necessary to take into consideration the influence of the auxiliary pulse signal on the following printing cycle. Thus, it is possible to shorten the printing cycle and to increase the print speed. Since it is not necessary to provide a circuit for turning ON/OFF the application of the auxiliary pulse signal, it is possible to reduce the cost of the driving circuit.
Another ink jet recording apparatus of the present invention includes: a head body provided with a nozzle and a pressure chamber, which is communicated to the nozzle and is filled with ink; an actuator provided in the head body and including a piezoelectric element and an electrode for applying a voltage across the piezoelectric element for applying a pressure on the ink in the pressure chamber; and a driving circuit for supplying an actuator driving signal to the electrode of the actuator, wherein: in every printing cycle, the driving circuit always supplies an auxiliary pulse signal for driving the actuator to a degree such that the ink is not discharged; if an ink discharge instruction signal instructing an ink discharge is received, the driving circuit supplies, after the auxiliary pulse signal is supplied, an ink discharge pulse signal for driving the actuator so that the ink is discharged; and a time T from a completion of the supply of the auxiliary pulse signal until a start of the supply of the ink discharge pulse signal is set to satisfy n*Tc+Tc/4xe2x89xa6Txe2x89xa6n*Tc+3Tc/4, where Tc is a Helmholtz period of a head, and n is zero or a natural number.
The ink discharge pulse signal may be made up of a plurality of pulses.
In this way, an increase in the viscosity of ink is suppressed even for those nozzles through which ink is not discharged for a long period of time not only when forming small dots but also when forming medium dots. Thus, when forming medium dots, it is possible to obtain an effect as that obtained when forming small dots.
Each of the auxiliary pulse signal and the ink discharge pulse signal may be a pulse signal for driving the actuator so as to first depressurize, and then pressurize, the pressure chamber.
In this way, each of the auxiliary pulse signal and the ink discharge pulse signal is a pulse signal having a so-called xe2x80x9cpull-push waveformxe2x80x9d. When such a signal is supplied, the volume of the pressure chamber first increases and then decreases, whereby an ink meniscus is first pulled into the nozzle, and then pushed back outward from the inside of the nozzle. This replaces ink in the vicinity of the nozzle opening, and discharges an ink droplet from the nozzle.
It is preferred that a time T from a completion of the supply of the auxiliary pulse signal until a start of the supply of the ink discharge pulse signal is set to satisfy n*Tc+Tc/4xe2x89xa6Txe2x89xa6n*Tc+3Tc/4, where Tc is a Helmholtz period of a head, and n is zero or a natural number.
Note that the term xe2x80x9cHelmholtz period of a headxe2x80x9d as used herein refers to the natural period of the entire vibration system including the ink (an acoustic element), the actuator, etc.
In this way, the vibration caused by the auxiliary pulse signal is more likely to be resonant with that caused by the ink discharge pulse signal, thus increasing the discharged ink volume.
It is preferred that: a pulse width of the auxiliary pulse signal is set to be xc2xc to xc2xd of a Helmholtz period of a head; and a peak value of the auxiliary pulse signal is set to be less than or equal to a value that is 0.6 times that of the ink discharge pulse signal.
In this way, the auxiliary pulse signal can be used as a signal that is very suitable for replacing ink in the vicinity of the nozzle opening without discharging an ink droplet.
The ink jet recording apparatus may further include: an ink jet head including at least the head body and the actuator; and a driving mechanism for relatively moving the ink jet head and a recording medium with respect to each other.
As described above, according to the present invention, it is possible to increase the discharged ink volume and to suppress an increase in the viscosity of ink in a nozzle. Therefore, it is possible to improve the ink discharging performance. For example, it is possible to prevent a non-uniformity in the print density in a solid print at the highest driving frequency, and to prevent a dot dropout or dot diameter variations during an initial ink discharging operation or during ink discharging operations at low driving frequencies. Moreover, since it is not necessary to provide a circuit for turning ON/OFF the application of the auxiliary pulse signal, it is possible to reduce the cost of the apparatus.